Anyone with a medical condition should inform the cruise line or your travel agent in advance. This will allow the ship's staff to add you to their on-board emergency list, which is vital in case of general emergency or if something happens to you.
Take out a travel insurance policy that will reimburse you for on-board medical visits as they can become very costly. Check the fine print on your insurance, they will not cover you if you have pre-existing conditions and have not disclosed them.
Most medical care on-board is perfectly adequate - they will have most of the common pills and prescription drugs, but at a heavy surcharge.